At some point during this pandemic or next, all countries will need to answer hard questions about whether and when scarce ICU resources (such as ventilators, beds and staff) should be either withheld or withdrawn from certain groups of patients solely for the purpose of providing them to others. Attempts to answer these hard questions can be found in ICU triage protocols and ethical guidance documents, many of which embrace the foundational principle of ‘save the most lives’. Unfortunately, this worthwhile goal has generated many suggestions that could violate the law. This article identifies 10 ways in which the withholding or withdrawal of a clinically indicated ventilator might violate a patient’s rights, along with recommendations on how to avoid doing so. While our analysis is based on UK law, its lessons are relevant for other countries with similar legal systems. If the issues we identify are not addressed, doctors may act unlawfully. Worse, patients may die unlawfully.